The disclosures of Japanese Patent Applications No. 2002-115337 filed on Apr. 17, 2002 and No. 2002-121902 filed on Apr. 24, 2002, each including the specification, drawings and abstract, are incorporated herein by reference in their entireties.
1. Field of the Invention
The invention relates to an evaporative fuel emission control system, and in particular to an evaporative fuel emission control system for processing fuel vapor generated in a fuel tank of an internal combustion engine, without releasing the fuel vapor to the atmosphere.
2. Description of Related Art
Conventionally, there is known an evaporative fuel emission control system including a canister for adsorbing fuel vapor generated in a fuel tank, as disclosed in, for example, Japanese laid-open Patent Publication No. 10-274106. The system disclosed in this publication includes a mechanism for purging fuel vapor adsorbed in a canister by utilizing flow of air, and a separation film for separating or isolating fuel vapor from the purge gas. The system further includes a condensing unit for condensing fuel vapor isolated by the separation film, and a return path through which the condensed fuel returns into the fuel tank. The evaporative fuel emission control system thus constructed is able to process evaporative fuel vapor generated in the fuel tank, within a closed system including the canister. Thus, the known system is able to effectively prevent release of fuel vapor into the atmosphere without requiring complicated control, such as correction of the fuel injection quantity of the engine.
The above-described known system, however, is not able to sufficiently condense fuel vapor only by means of the separation film. Therefore, the known system includes the condensing unit for further condensing and liquefying evaporative fuel gas produced as a result of condensation by the separation film. If the use of the separation film alone can provide a sufficiently high condensing capability, on the other hand, the system may be constructed such that the evaporative fuel gas produced through condensation by the separation film is caused to flow into the fuel tank as it is. With this arrangement that requires no condensing unit, the system can be simplified, and the cost of manufacture of the system can be reduced.
In the meantime, when no fuel vapor is purged from the canister, namely, when no purge gas flows through the system, gas containing no fuel vapor but mainly consisting of air may accumulate on the upstream side of the separation film. Therefore, even if the separation film exhibits excellent condensing capability, processed gas whose fuel concentration is not sufficiently increased may be produced on the downstream side of the separation film immediately after purge gas starts flowing through the system.
If the processed gas having such a low concentration of fuel flows directly into the fuel tank, air contained in the gas may not be sufficiently dissolved in the fuel. Then, the presence of undissolved air may cause problems, such as vapor lock of a fuel feed pump or introduction of bubbles into fuel to be injected into the engine.
It is also to be noted that in the known system as described above, the separation film needs to be maintained in an appropriate condition so as to process gas containing fuel vapor. Thus, it is desirable to immediately detect an abnormality in the separation film, so as to ensure the intended functions of the system.
It is therefore an object of the invention to provide an evaporative fuel emission control system that has a function of condensing fuel vapor by using a separation film(s), and is also able to prevent a large amount of air from flowing into a fuel tank immediately after start of flow of purge gas.
To accomplish the above object, there is provided according to a first aspect of the invention an evaporative fuel emission control system, which comprises (a) a canister that adsorbs fuel vapor generated in a fuel tank, (b) a canister outgoing gas producing unit that causes a canister outgoing gas to flow out of the canister, (c) a vapor condensing unit that condenses the canister outgoing gas to provide a processed gas containing a higher concentration of fuel vapor than that of the canister outgoing gas, (d) a processed gas passage through which the processed gas is fed to the fuel tank, and (e) a controller that restricts flow of the processed gas into the fuel tank when the fuel vapor concentration in the processed gas is lower than or is expected to be lower than a predetermined level.
In the control system constructed as described above, when the fuel concentration in the processed gas is lower than the predetermined level or is expected to be lower than the predetermined level, flow of the processed gas into the fuel tank is restricted. It is thus possible to avoid problems that would occur if processed gas having a low concentration of fuel vapor is collected in the fuel tank.
According to a second aspect of the invention, there is provided an evaporative fuel emission control system, which comprises (a) a canister that adsorbs fuel vapor generated in a fuel tank, (b) a canister outgoing gas producing unit that causes a canister outgoing gas to flow out of the canister, (c) a vapor condensing unit that condenses the canister outgoing gas to provide a processed gas containing a higher concentration of fuel vapor than that of the canister outgoing gas, (d) a processed gas passage through which the processed gas is fed to the fuel tank, (e) a bypass passage that allows communication of an upstream side of the vapor condensing unit with the fuel tank, (f) a switching valve having an open state in which the bypass passage communicates the upstream side of the vapor condensing unit with the fuel tank, and a closed state in which the bypass passage is shut off, and (g) a switching valve controller that controls the switching valve such that the switching valve is placed in the open state during stop of the canister outgoing gas producing unit, and is placed in the closed state during an operation of the canister outgoing gas producing unit.
In the control system as described above, fuel vapor in the fuel tank is guided to the upstream side of the vapor condensing unit through the bypass passage during stop of the canister outgoing gas producing unit. Thus, a portion of the system at the upstream side of the vapor condensing unit can be filled with evaporative fuel gas having a high fuel concentration even under a situation where no canister outgoing gas flows through the system. It is therefore possible to produce processed gas having a sufficiently high fuel concentration immediately after start of an operation of the system.
According to a third aspect of the invention, there is provided an evaporative fuel emission control system, which comprises (a) a canister that adsorbs fuel vapor generated in a fuel tank, (b) a canister outgoing gas producing unit that causes a canister outgoing gas to flow out of the canister, (c) a vapor condensing unit that condenses the canister outgoing gas to provide a processed gas containing a higher concentration of fuel vapor than that of the canister outgoing gas, (d) a processed gas passage through which the processed gas is fed to the fuel tank, and (e) a canister heater that heats the canister. With this arrangement, the canister is heated by the canister heater so that fuel vapor can be purged from the canister at an improved efficiency.